Systems and apparatus for on-demand water heating are known. One common system uses a large tank of heated water from which to provide hot water. Some systems use mechanical thermostatic mixing valve that can fail and that require hotter fluids than needed to be mixed with cooler fluid in order to be able to provide somewhat accurate output temperatures.
Tankless systems have been designed to use less energy without heating a large volume of water. Tankless water heaters heat water directly without use of a storage tank which avoids standby heat loss. Tankless water heating products do not function well in environments with intermittent or constant start/stop of water flow. In these conditions, the ability to regulate the water temperature for immediate use is severely compromised—rendering tankless water heaters less effective. Also, tankless systems alone do well with constant flow for long periods of time.
U.S. Pat. No. 5,233,970 discloses a semi-instantaneous water heater with a helical heat exchanger. The water heater generates domestic hot water by transferring heat from the circulating fluid of a modulating boiler. It is particularly suited for use in a combination system, which provides both space and water heating. The semi-instantaneous design incorporates a small cylindrical tank containing stored hot water and an immersed heat exchanger. The heat exchanger is a helical coil disposed in the annular space between two metal sheets that have been rolled into cylinders. The coil conveys heated fluid from the boiler. Heat from the coil is transferred to the water, which is admitted to the tank via the helical passageway formed by the two sheets and the inter-coil space of the helix. The heat exchanger effectively transfers heat by forced convection at a high rate when required by a high flowrate of water. Its disposition in the tank also permits good heat transfer by free convection to quiescent water in the tank when this heating mode is required. The stored volume of hot water provides thermal capacitance to meet brief draws of hot water without short period on/off cycling of the boiler. It also aids in maintaining temperature stability when the hot water flowrate is turned up or down. The small size of the tank allows for effective thermal insulation, thereby minimizing heat loss.
U.S. Patent Publication 20100086289 discloses a modular tankless water heater apparatus with precise power control circuitry designed for use in a system including a water supply conduit and a hot water conduit. The apparatus includes a heating tube assembly with a plurality of tubes positioned in parallel juxtaposition and connected adjacent to the ends into a series connected configuration to form a continuous fluid passage. A heating element is enclosed in each tube and extends between the ends with each heating element including an electrical connector and an electrical control. A programmable electrical power controller is connected to the electrical controls of the heating elements and to flow sensor and heat sensor apparatus positioned in the continuous fluid passage. The controller is programmed to activate the electrical controls one at a time in response to a demand signal from the flow sensor and heat sensor apparatus.
Various control systems are available for water heating systems. A proportional—integral—derivative controller (PID controller) is a control loop feedback mechanism (feedback controller) used in industrial control systems. A PID controller calculates an “error” value as the difference between a measured process variables and a desired set point. The controller minimizes the error by adjusting the process control inputs. The PID controller calculation (algorithm) involves three separate constant parameters, and is accordingly sometimes called three-term control: the proportional, the integral and derivative values, denoted as P, I, and D.
U.S. Patent Publication 20080285964 discloses a modular heating system for tankless water heater for heating water passing therethrough. The tankless water heater includes a control module with a controller and a heating system, each of which are configured in a modular/separate arrangement. The heating system includes an inlet portion, an outlet portion, and a modular heater interconnected therebetween. The modular heater comprises a plurality of heating units, each heating unit comprising a heating tube and a coupler, wherein each heating tube defines an interior region and each heating tube includes a helical structure whereby the helical structure imparts a swirling motion on water passing through the interior region of the tube. A heating element is also disposed within the interior region of the heating tube, and electric power applied to the heating element acts to heat the water passing through the tube. A first temperature sensor may be positioned so as to detect water temperature proximate the inlet portion, and the first temperature sensor is in communication with the controller. Also, a second temperature sensor positioned so as to detect water temperature proximate the outlet portion, and the second temperature sensor is in communication with the controller. Additionally, a flow meter is positioned proximate the inlet portion, and the flow meter, which detects fluid flow (and thereby fluid volume), is in communication with the controller. The controller, receiving the signals from the temperature sensors and the flow meter, directs signals to switches positioned at each tube so as to apply electric current to the heating elements.
Publication WO2009020659 discloses a tankless water heater comprising a pipe, at least one coil around the pipe, at least one heating element located within the pipe and responsive to an electromagnetic field generated by the coil, and a controller to apply an alternating current (AC) signal to the at least one coil, the AC signal applied at a predetermined frequency and magnitude to cause the heating element to heat water flowing in the pipe to a predetermined temperature through induction heating.
Publication WO2006101326 discloses an apparatus and method for controlling temperature of a hot-cold water purifier. The apparatus for controlling temperature includes a display/control unit having indicators for indicating detected and reference temperatures, a hot water switch and a cold water switch, a hot water temperature sensor, a cold water temperature sensor, a controller, a heater and a cooler. The method for controlling temperature controls hot and cold water temperatures of the hot-cold water purifier. The apparatus and method can perform hot and/or cold water power-on/off as well as set reference temperatures to multiple levels desirable to the user, thereby more precisely controlling the temperatures.